Project Summary This project seeks to develop a high density, minimally invasive optical micro?ber array for long-term recording and manipulation of brain activity. Optical methods have become a cornerstone of modern brain science in animal models, and hold great potential for future human prosthetic devices. However, light scattering severely limits optical approaches for deep brain recording and stimulation. Current photometry methods of implanting optical ?bers into deep brain areas work with relatively large ?bers designed for the communications industry (125 ?m). This project builds an optical micro?ber array to record from and stimulate deep brain areas. The device achieves a high channel count with sub-cellular (7 ?m) optical micro?bers distributed in three- dimensional volumes of the brain. To implant the device, individual micro?ber light guides are bundled together, strengthening each ?ber through mutual support. During insertion into the brain, the bundle of micro?bers splays and each micro?ber follows a distinct path into the brain as it is de?ected by tissue inhomogeneity. This process is hypothesized to preserve the minimally invasive properties of a single 7 ?m ?ber. Prototype designs reveal healthy neurons in close proximity to the implanted micro?bers, and high signal to noise recordings in vitro. The project builds on preliminary data to test high channel count devices for both recording and stimulation. To advance this technology, the project involves a series of aims to characterize tissue response to high channel count implants, develop a rotary ?uorescence microscope to interface with the array, and benchmark the performance of the device for both recording and stimulation of genetically encoded constructs in deep brain regions.